Difficulties have been encountered in the manufacture of poly(ethylene terephthalate) by the ester interchange reaction. Obviously, highly purified dimethyl terephthalate and highly purified glycol are preferred starting materials in order to form a high quality product. However, even these highly purified materials are very sluggish with respect to ester interchange and in the case of less purified materials the reaction is too slow for practical commercial operation. Because of this slow rate of reaction, it has been found essential, in commercial operation, to employ a suitable catalyst to speed up the reaction.
Many catalysts have been proposed for the ester interchange and polycondensation reactions in the manufacture of poly(ethylene terephthalate). These catalysts have not proven to be entirely satisfactory as fibers, films and plastic parts produced from the condensation polymers using these catalysts do not possess the desired whiteness or lack of objectionable color and/or the desired stability.
It is well known in the art that thermoplastic polyesters comprising ethylene glycol terephthalate regularly recurring units constitute valuable starting materials for the packaging of food products, especially in the form of bottles intended for storing carbonated beverages. One of the essential properties is that a catalyst be used to provide a transesterification and polycondensation rate to be commercially attractive and to be fast enough to minimize by-products and side reactions which are inherent with slow transesterification and polycondensation reactions in poly(ethylene terephthalate). Another essential property which packaging material fabricated from polymeric substance must possess is the absence of any compound or additive which is capable of migrating into the food product or drink and of impairing its taste or smell. It is known that poly(ethylene terephthalate) and copolymers of poly(ethylene terephthalate) resins release acetaldehyde by degradation and thus possess a strong smell and a characteristic taste which appear at extremely low concentrations. The aforesaid problem is particularly significant and acute in packaging (bottling) of mineral water and carbonated beverages. Thus, polyester bottles intended for aerated mineral water and carbonated beverages must have a very low acetaldehyde concentration. The minimum level of acetaldehyde required in a polyester bottle is set by the manufacture of a given product and the minimum can be higher or lower depending on the specific product being bottled. Therefore, any process or technique which will further reduce the acetaldehyde concentration in a polyester or copolyester bottle would be desirable and an advantage to those who package their products in polyester bottles. The amount of acetaldehyde present in the finished shaped articles depends both on the residual amount present in the polyester granules, before they are converted, and on the amount formed during the conversion in the molten state, as a result of degradation under the conditions of shear and temperature which are required for molding.
This invention is related to a specific catalyst system for poly(ethylene terephthalate) and copolyesters of poly(ethylene terephthalate) produced from dimethyl terephthalate. The catalyst comprises a titanium/alkali metal or alkaline earth metal complex, along with certain other conventional catalyst-inhibitor materials. The polyester produced has a fast polycondensation rate suitable for commercial production of polyester and has a reduced acetaldehyde generation rate as measured by head space analysis.
There are numerous techniques and processes in the literature for the reduction of acetaldehyde generation rate in poly(ethylene terephthalate), the reduction of acetaldehyde in the finished article, and the thermal stabilization of polyesters to prevent or reduce the amount of acetaldehyde produced during the process or forming the finished article. There are also many examples in the literature where alkaline or alkali metal salts are included in the reaction mix of poly(ethylene terephthalate) and poly(ethylene terephthalate) copolymers to achieve certain desirable effects and improvements in polymer properties, none of which disclose the use of alkali and alkali metal salts to reduce the quantity of acetaldehyde produced when the polymer is used as a packaging material. Also, there are only a few disclosures of titanium catalyst systems being used as a catalyst system for poly(ethylene terephthalate) which produces a useful product and these are mostly for films and fibers and not for packaging materials.
The present invention is based on a catalyst stabilizer system containing compounds of (1) manganese, zinc or calcium, (2) antimony, (3) cobalt, (4) phosphorus, (5) titanium and (6) alkali or alkaline earth metal salts.
It is well known in the art that the titanium catalyzed reaction of dimethylterephthalate and ethylene glycol provides the fastest polycondensation rates of any known catalyst for the preparation of poly(ethylene terephthalate). It is also well known in the art that titanium catalyzed poly(ethylene terephthalate) produces a yellow colored polymer. Therefore, to be a useful product, titanium catalyzed poly(ethylene terephthalate) must contain a color inhibitor or stabilizer such as phosphorus compounds and/or cobalt compounds or blue toner pigments to provide sufficient color stabilization to make titanium catalyzed poly(ethylene terephthalate) a commercially acceptable product.
U.S. Pat. No. 3,907,754 discloses a catalyst-inhibitor system containing titanium, manganese, cobalt, phosphorus for poly(ethylene terephthalate) which provides a fast reaction (production) rate and polymer with good color. This patent does not disclose antimony or an alkali metal salt as part of this catalyst combination, and does not address acetaldehyde generation. U.S. Pat. No. 3,962,189 discloses manganese, titanium, cobalt, phosphorus plus an alkali metal salt which is used to produce poly(ethylene terephthalate) with fast reaction rate, good color and an alkali metal salt in low concentration (2 to 32 ppm) to prevent a batho-chromic dye shift when the polyester is spun into fiber and dyed. This patent does not include antimony in the catalyst inhibitor system, and does not address acetaldehyde generation. U.S. Pat. No. 4,010,145 discloses a manganese, titanium, cobalt, phosphorus antimony catalyst inhibitor system for producing poly(ethylene terephthalate) at a fast rate with good color. There is no alkali metal salt disclosed in this patent and does not address acetaldehyde generation. U.S. Pat. No. 4,356,299 discloses a titanium, antimony, manganese, cobalt, and phosphorus catalyst system for producing poly(ethylene terephthalate) at fast rate with good color. This patent is for a reduced titanium concentration to provide good color and fast reaction rate, and does not address acetaldehyde generation. U.S. Pat. No. 4,357,461 discloses the use of an alkali metal salt of ethylenediamine-tetra acetic acid to reduce the acetaldehyde generation rate inherent in preparation and processing of poly(ethylene terephthalate) for packaging. However, this patent does not address the use of a titanium catalyzed polyester. U.S. Pat. No. 4,361,681 discloses the use of dicarboxylic acid anhydrides to reduce acetaldehyde in the preparation and processing of poly(ethylene terephthalate) for packaging and uses, namely bottles. This patent addresses the post treatment of previously prepared polymer to reduce hydroxyethyl ends and thus reduce acetaldehyde.